A method has been desired for protecting automobiles and their parts which have been coated against the attack by various suspended matters such as dust and rainwater or other harmful matters that can damage, dull or discolor the paint film during shipping on truck or ship to overseas or other remote places.
Conventionally, a method of coating a coating material mainly comprising a wax in a thickness of 5 to 40 .mu.m has been known as such a countermeasure. However, this method has some disadvantages. For example, it is difficult to form a wax film at a uniform thickness, making it impossible to obtain a uniform protection; a wax film is easily stained; a wax film is weak to acidic rainwater; wax or other matters penetrate in coated paint film to cause discoloration; much time and labor are required to apply and remove the wax film; and this method requires the use of a solvent and the disposal of a waste liquid, causing environmental pollution.
On the other hand, various surface protective sheets comprising a support and an adhesive layer formed thereon have been known. A sheet comprising a support having formed thereon a radiation-curing adhesive layer having a lowered glass transition point or a rubber adhesive layer made of a polyisobutylene or the like has been proposed as a surface protective sheet for an adherent having a paint film, as disclosed in, for example, JP-A-2-199184 and JP-A-6-73352. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). This sheet can overcome the above-described problems in the prior art. However, this approach has the disadvantage that when the protective sheet which has been applied to protect the surface of a paint film is peeled off, the paint film may deteriorate or discolor, causing damaged portions, and the protective sheet may not easily be removed.
As a result of extensive studies to overcome the above-described problems, it was found that the deterioration or discoloration of the paint film is due to that water or water vapor is present between the protective sheet and the paint film in adhering the protective sheet to the paint film and is sealed therebetween over a long period of time. For water or water vapor which can easily be perceived, such as large water droplet (e.g., rainwater), the presence of water or water vapor over a long period of time can easily be avoided by a wiping treatment before adhering the protective sheet or a re-adhering treatment of the protective sheet after adhering the protective sheet although such a treatment requires additional time and labor. However, even if water or water vapor which is difficult to perceive, such as mist-like water droplets, is present, the same discoloration of the paint film occurs. Thus, it is extremely difficult to completely avoid the presence of water or water vapor between the protective sheet and the paint film.